1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, or the like and, more specifically, to a tandem image forming apparatus.
2. Description of the Related Art
In this type of image forming apparatus, a plurality of image bearers are arranged along the direction of a surface movement of a transfer member such as a recording member or an intermediate transfer member. An image is formed by forming toner images on the image bearers by using a developing agent (hereinafter, “developer”) provided in a developing device and subsequently transferring the formed toner images onto the transfer member. In the image forming apparatus configured in this manner, the toner images formed on the image bearers are transferred onto the transfer member by applying a transfer bias thereto from a transfer unit. The percentage of the toner (hereinafter, “transfer rate”) that is transferred onto the transfer member out of the toner structuring each of the toner images formed on the image bearers changes in relation to an electric charge amount (hereinafter, “charge amount”) of the toner and the magnitude of the transfer bias.
While the developer deteriorates over the course of time, the toner charge amount (more specifically, a specific charge (Q/M) expressed as a charge amount per unit mass of the developer) changes. Thus, even if the transfer bias is set to an appropriate value in accordance with the toner charge amount at an initial point in time when the developer had not yet deteriorated, the transfer bias no longer has an appropriate value at a later time when the developer became deteriorated, because the toner charge amount has changed.
An image forming apparatus disclosed in Japanese Patent No. 3,172,557 is configured to count the number of sheets printed, to find out the degree of deterioration of the developer in accordance with the counted result, and to correct the transfer bias so as to obtain an optimal transfer current. When the image forming apparatus disclosed in Japanese Patent No. 3,172,557 is used, even if an overall toner charge amount has decreased due to deterioration of the developer over the course of time, because the transfer bias is corrected in accordance therewith, it is possible to inhibit degradation of image quality caused by the deterioration of the developer over the course of time.
It has been found out, however, that even if the number of sheets printed is the same, the degree of deterioration of a developer varies depending on output conditions of the image forming apparatus. For example, under a condition where a large number of sheets are output with images having a low image area ratio such as diagrams or text images, the developer tends to deteriorate faster than under a condition where a large number of sheets are output with images having a high image area ratio such as solid images, even if the numbers of sheets printed are the same. It is considered that the reason is that, when the large number of sheets are output with the images having a low image area ratio, because the consumption amount of the developer in the developing device is smaller, a larger amount of developer stays inside the developing device, so that a larger stress is applied to the developer, and the deterioration of the developer thus progresses faster.
Accordingly, when the transfer bias is corrected by finding out the degree of deterioration of the developer simply on the basis of only the number of image formed sheets, it is difficult to set the transfer bias to an appropriate value because the degree of deterioration of the developer varies depending on the output conditions that have so far been used by the image forming apparatus.
Thus, there is a need for an image forming apparatus capable of finding out the degree of deterioration of the developer more appropriately and setting the transfer bias to an appropriate value.